(1) Field of the Invention
The present invention relates to a gas chromatographic method and apparatus which uses a primary separation column and parallel secondary separation columns to separate volatile organic compounds (VOCs) in a sample at different times as a result of the sample flowing through the columns. In particular, the present invention relates to a method and apparatus which uses a valve which accumulates a sample from the primary column for transfer to the secondary columns in parallel. The primary column has a smaller fluid flow capacity than the combined fluid capacities of the secondary columns. In this manner, the chromatographic separations of the primary and secondary columns are matched to provide the best available separation of compounds in the sample.
(2) Description of Related Art
Volatile organic compounds (VOCs) are key components in industrial, environmental, and medical samples. Air, water soil, and body fluids often contain hundreds of VOCs (Helmig, D., et al., Chemosphere 38 2163-2187 (1999); and Phillips, M., et al., Chromatogr. B 729 75-88 (1999)) with concentrations in the part-per-trillion to part-per-million range (Helmig, D., et al., Chemosphere 38 2163-2187 (1999); and Phillips, M., et al., Journal of Chromatography B 729 75-88 (1999)) Gas chromatography/mass spectrometry (GC/MS) is conventionally used to characterize complex VOC mixtures. However, comprehensive two-dimensional gas chromatography (GCxc3x97GC) has recently emerged as an alternative to GC/MS (Liu, Z. Y., et al., J. Chromatogr. Sci 29 227-231 (1991); Beens, J., et al., J. Chromatogr. A 919 127-132 (2001); Bruckner, C. A., et al., Anal. Chem. 70 2796-2804 (1998); Frysinger, G. S., et al., J. High Res. Chrom. 22 195-200 (1999); Kinghorn, R. M., et al., High Res. Chrom. 21 620-622 (1998); and Lewis, A. C., et al., Nature 405 778-781 (2000)).
GC/MS instruments use gas chromatography to separate mixtures into individual components and mass spectrometry to detect and identify each component. Chromatographic separation is the rate-limiting step: complex samples often require more than 30 minutes to resolve.
Chromatography is an analytical method for the separation and identification of chemical compounds from mixtures. Gas chromatography is a particularly well-known discipline of this science which, when used in combination with quantitative instrumentation (for example GC-IR, GC-UV, GC-MS), provides the user with reliable results. However, in an attempt to find lower cost methods and decrease sample testing time, a variety of sources have taught the technique of comprehensive two-dimensional gas chromatography as a high resolution alternative to the previously known methods. In comprehensive two-dimensional GC, a sample is first injected into and separated by a primary column. Thereafter, at least a portion of the separated sample is collected and injected into a secondary column for further separation.
Comprehensive two-dimensional gas chromatography (GCxc3x97GC) is a high resolution alternative to GC/MS. GCxc3x97GC has been developed in a limited number of laboratories over the past 10 years (Phillips, J. B., et al., J. Chromatogr. A. 856 331-347 (1999); and Bertsch, W., J. High Resol. Chromatogr. 23 167-181 (2000)). GCxc3x97GC subjects the entire sample to two serial chromatographic separations. The sample is first partially separated by a primary column. Sample components are collected by a modulator as they leave the primary column and are subsequently injected at regular intervals into a secondary column where they undergo a fast secondary separation. The stationary phases of the primary and secondary columns have different selectivities so that species that co-elute from the primary column can be separated by the secondary column. GCxc3x97GC frequently produces greater levels of chromatographic separation than GC/MS (Bertsch, W., J. High Resol. Chromatogr. 23 167-181 (2000) and Seeley, J. V., et al., Analytical Chem. 72 4346 (2000)).
U.S. Pat. No. 5,196,039 that issued to Phillips et al provides an example of prior art two-dimensional GC. Specifically, Phillips et al disclose a method and apparatus for two-dimensional chromatography wherein a sample is injected and separated in a first column (a first dimension), collected, and then re-injected and separated into a second column (a second dimension) that is in series with the first dimension.
Objects
It is an object of the present invention to provide a comprehensive gas chromatographic method and apparatus which speeds up the time of separation and increases the resolution of a sample. It is further an object of the present invention to provide an apparatus which is relatively economical to assemble and to use compared to prior art methods. These and other objects will become increasingly apparent by reference to the following description and the drawings.
The present invention relates to a comprehensive two-dimensional gas chromatograph apparatus, said apparatus comprising a primary column, said primary column interacting with a sample to provide a first dimension result; a first secondary column and a second secondary column, said sample interacting with each of said first and second secondary columns to provide a pair of second dimension results; and a valve, said valve providing fluid communication of said sample from said primary column to each said first and second secondary columns, whereby said secondary columns are in a parallel arrangement through a single connection to the valve.
In a preferred embodiment primary column is nonpolar. Further, preferably the first and second secondary columns are polar. Preferably the first secondary column has a selectivity different from second secondary column. Preferably the single connection is through a Y connection with two arms connected to the secondary columns and a leg connected to the valve.
The present invention also relates to a comprehensive two-dimensional gas chromatograph apparatus, said apparatus comprising a primary non-polar column, said primary column interacting with sample to provide a first dimension result; a first polar secondary column and a second polar secondary column, said sample interacting with each of said first and second secondary columns to provide a pair of second dimension results; and a valve, said valve providing fluid communication of said sample from said primary column to each said first and second secondary columns, whereby said secondary columns are in a parallel arrangement through a single connection to the valve.
The primary column has a first flow capacity smaller than the combined second and third flow capacities of the secondary columns and wherein the valve accumulates the sample for transfer to the secondary columns through the single connection to the valve. The ratio of second and third flow capacities combined to primary flow capacity is preferably between about 10 to 1 and 30 to 1. Preferably the ratio of second and third flow capacities to primary flow capacity is about 26.6 to 1. Preferably the primary column has a first flow capacity smaller than the combined second and third flow capacities of the secondary columns and wherein the valve accumulates the sample for transfer to the secondary columns through the single connection to the valve. Preferably the single connection is through a Y connection with two arms connected to the secondary columns and a leg connected to the valve.
The present invention also relates to a method for comprehensive two-dimensional gas chromatography comprising the steps of: injecting a sample into a primary column to obtain a first dimension; communicating said sample from said primary column through a valve; injecting at least a portion of said sample from said valve through a single connection simultaneously into a first and a second secondary column to obtain a pair of second dimensions.
The primary column has a first flow capacity smaller than the combined second and third flow capacities of the secondary columns and wherein the valve accumulates the sample for transfer to the secondary columns through the Y. Preferably the ratio of second and third flow capacities to primary flow capacity is between about 10 to 1 and 30 to 1. Preferably the single connection is through a Y connection with two arms connected to the secondary columns and a leg connected to the valve.